Integral hydraulic power steering gears are commonly used in trucks, heavy equipment such as earth-moving vehicles, and construction vehicles. “Integral” refers to a steering gear containing a manual steering mechanism, a hydraulic control valve assembly, and a hydraulic power cylinder integrated into a single unit.
The hydraulic power cylinder typically comprises a chamber divided into two chamber portions by a piston. The piston has a set of teeth which mesh with a sector gear fixed to an output shaft. The output shaft is connected via steering linkage to steerable wheels of a vehicle to steer the vehicle when the output shaft is rotated.
The hydraulic control valve assembly controls flow of pressurized hydraulic fluid between a hydraulic pump and one of the chamber portions to control the direction and amount of steering. One type of control valve assembly includes a closed center valve. In such a system, the hydraulic flow to the two chamber portions is blocked by the valve, when the steering wheel is centered and no steering of the steerable wheels is underway.
The valve assembly typically comprises two relatively rotatable valve elements, one of which is connected to a rotatable input shaft operatively coupled to the vehicle steering wheel. The other valve element is connected with a follow-up member, such as a ball screw drive, which rotates in response to movement of the piston. The ball screw drive provides a direct connection between the input shaft and the piston to allow for manual steering of the vehicle in the event of hydraulic fluid pressure loss.
Associated with the shaft can be an electric motor, which is constructed to resist turning of the shaft by the driver of the vehicle. This resistance provides the driver a steering “feel”, if the force from the hydraulic powered gear makes the steering wheel too easy to turn by the driver.